Diagnose and Repair a Yaesu FT-7800 Ham Radio

[Alan Wolke] aka [w2aew] was challenged to repair a friends Yaesu FT-7800 ham radio. This radio operates on two ham bands, 2 m VHF and 70 cm UHF. The complaint was that the 2 m side was not working but the 70 cm was transmitting fine. Alan started by verifying the complaint using a Bird watt meter with a 50 watt slug and terminating the signal into a 50 W dummy load. [Allen’s] bird meter is the type that has an RF sampler that can be connected to an oscilloscope for added signal viewing and validation.

After verifying that the radio was not working as described, Alan starts by glancing over the circuit board to look for any obvious damage. He then walks us through a block diagram as well as a circuit diagram of the FT-7800 radio before stepping us through the troubleshooting and diagnostics of radio repair. Even when he realizes he might have found the problem he still steps us through the remainder of his diagnostics. The skills and knowledge that Alan shares is extremely valuable to anybody looking to repair radios.

Spoiler alert. At the end of the first video he determines that the pin diodes near the final VHF output were bad. In the second video he reveals that he could no longer source these bad components. Through some clever evaluation of a more current Yaesu radio, [Allen] was able to find suitable replacement components. Lesson two ends with some surface mount solder rework tips as well as testing that the repair was successful.

And just in case you don’t know what a pin diode is, or is used for, Alan shares a third video covering just what this component is and does in a radio. You can follow the jump to watch all three videos.

Well, even though this isn’t a hack per se (except for the substitution of the PIN diode part numbers), hopefully it does present some troubleshooting and rework techniques that all hakers/makers can benefit from.

So many people here are quick to cry “not a hack!” This is unfortunate, and rather misguided. The term “hacker” (according to some lore) originates from noise made by rapid keyboard typing shown by those with considerable skill. The skill shown in this video, like everything, is less than some and more than others. Considering the number of “[turn on and off a device] with an Arduino” and articles accepted as hacks, this one repairing radios should by far be considered a hack, and a mighty good one at that.

Alan, your explanation of using PIN diodes as RF switches was most interesting. Thank you.

While I agree with you about spending hundreds of dollars on repairs, it’s still not a hack. A “hack” is fundamentally repurposing something for which it wasn’t originally intended. This is a repair. That does not mean it is without value, as many people, hackers or otherwise, don’t fully understand proper troubleshooting and repair techniques.

Does the post have value? Without question. Is it useful? To many, yes. Is it a hack? No.

I think from looking back at HaD posts – you don’t even need to go far – you can see that they post things that are of interest to the hacking community without necessarily being hacks. Think of it as a blog for hackers, not a just blog of hacks.

The significant issue is understanding how things work. Repair with or without access to exact replacement parts is the ultimate expression of this.

I’m far more proud of repair work like driving home in a VW on three cylinders after melting a hole in #3 (I pulled the pushrods out) or tracking down the bad solder joint in a 60 MHz analog scope horizontal circuit than I am the myriad of things I’ve cobbled together from the bits and pieces I had lying around. The ultimate being the TR7 engine taken apart by someone else I reassembled from a pile of parts with no manual. All I knew was it was an internal combustion engine. Before that I “worked” on cars. After that I was a “mechanic”.

There are plenty of people who can “hack” things they can’t repair, but anyone who can repair something can do anything they want with the pieces.

Slightly unfair since he had a schematic and a layout and a block diagram. And all those handy ground points right next to the probe points.
But still not bad to watch though.
It’s just that so often it’s impossible to get any info on commercial devices.

Companies that make ham stuff are pretty good about getting schematics out there. There’s a lot of experimentation and customization in the community – and they know it. I’ve even seen some radios where a logic map for the microcontrollers is provided.

I think the dual band is not attached to the motherboard so, if you intend to release the fotons of the card you MUST seal the Hijack shield and create an IFGJP shard into it in order to validate the code

The finals are the first thing you should check, as they are generally the first thing to give up the magic smoke when a radio without SWR protection circuitry is connected to a mismatched load(or not connected to a load at all) and transmits with power.